The present invention relates to polymeric compositions for channel type secondary electron multiplier tubes and plates or targets and a method for manufacture thereof.
The channel type secondary electron multiplier tubes have been widely used instead of the conventional separate dynode type multipliers mainly for the detection of charged particles, photons and X-rays. In general they are made of lead glass or ceramic and have an inner wall layer capable of emitting secondary electrons and having a suitable resistance. Secondary electrons emitted from the inner surface are accelerated in a high electric field so that the secondary electron emission effect may be cascaded to multiply the secondary electrons. Presently available in the market are thin-film type secondary electron multiplier tube made of a lead glass envelope with the inner wall alone capable of being semiconductive and emitting secondary electrons, and a bulk type multiplier tube wherein an envelope itself is made of a material having a suitable degree of semiconductivity and capable of secondary electron emission, the material being for instance ZnTiO.sub.3 or BaTiO.sub.3. Depending upon the designs, the secondary electron multiplier tubes may be divided into a linear type and a circular type. The linear type is susceptible to "ion feedback"; that is, the effect caused by the ionization of the gases in the vacuum, and tends to encounter many difficulties in practice. The circular type has been widely used, but the manufacture of the circular type electron multiplier tubes involves many problems. The advantages of the secondary electron multiplier tubes are compact size, light weight, high gain and low noise and are especially apparent of when the multiplier tubes are used in the pulse counting mode. However, they also have disadvantages in that they are very expensive and tend to be easily broken because of their construction of fragile materials.
To overcome these problems, electron multiplier tubes made of polymeric compositions capable of secondary electron emission have been devised so that their excellent moldability and flexibility may be fully utilized. An electron-conductive polymeric composition is molded into a tube to provide a bulk type flexible channel electron multiplier (FCEM) tube having high gain. In order to eliminate the adverse effect of ion feedback, the electron multiplier tubes may be curved with a suitable radius. In addition, they exhibit strong resistance to mechanical and acceleration impacts so that they can be mounted on the artificial satellites for detecting charged particles and photons.
The flexible channel electron multipliers (FCEM) are made of electron-conductive polymeric compositions having volume resistivity between 10.sup.5 and 10.sup.10 ohm-cm. In general, aliphatic polymers have a maximum secondary emission yield .delta. max higher than that of the aromatic polymers, and the higher the ionization potential of a solid polymer, the higher the secondary yield .delta. becomes and the maximum secondary yield .delta. max is obtained when the primary energy Epmax is as low as 200 to 300 eV. That is, the secondary yield .delta. is relatively high when the primary energy is less than 200-300 eV. As a result, an electron which is caused to repeatedly strike the inner wall of a multiplier tube at a predetermined acceleration voltage has a higher number of bombardments or strikings and a relatively high yield. The gain is expressed by
G = .delta..sup.n
so that a high gain of 10.sup.8 is obtained at an acceleration voltage of 3 kV.
In general, the electron-conductive polymeric composition for an electron multiplier may be divided into (I) granularly dispersed polymeric compositions in which carbon black, graphite or metal are granularly dispersed in an insulating polymer having a relatively high secondary yield, (II) molecularly dispersed polymeric compositions wherein an organic semiconductor is molecuraly dispersed or dissolved in an insulating polymer having a relatively high secondary yield and (III) organic semiconducting polymers having inherent secondary electron emission capability. These compositions are disclosed in U.S. Pat. No. 3,808,494, British Pat. No. 1,288,816, Canada Pat. No. 883,443 and German Pat. No. 1,964,665 all of which are assigned to the present applicant. For instance, a secondary electron multiplier made of the compositions disclosed in these patents has a high gain of 10.sup.8 at an acceleration voltage of 3 kV, but subsequent analyses showed that the count rate dependency of gain varies widely, as will be described below.
The secondary electron multiplier tubes made of the granularly dispersed polymeric compositions (I) have poor count rate dependence. More particularly, with increase in count rate, the gain starts to decline at a count rate considerably lower than the theoretical critical rate so that saturation of output current occurs. However the secondary electron multiplier tubes made of the molecularly dispersed polymeric compositions (II) or organic semiconducting polymers (III) exhibit excellent count rate dependence substantially similar to the theoretical one. That is, the gain will not decrease even at a high count rate so that a large output current may be obtained. However, few of these compositions (II) or (III) have a sufficiently high volume resistivity of 10.sup.5 -10.sup.10 ohm-cm and desired moldability. Furthermore their heat deterioration is quick and they generate gases upon decomposition. Thus there has been a problem that the electron multiplier tubes cannot be easily made of these compositions (II) or (III).